1. Field of the Invention
The present invention relates to Schmitt Trigger circuits and more particularly to Schmitt Trigger circuits utilizing CMOS (Complementary Metal Oxide Semiconductor) circuitry.
A Schmitt Trigger is a circuit that provides an output signal of either logic 0 or logic 1 in response to an input signal. When the input signal crosses a specified threshold voltage (or current), the output changes to the other logic condition. When the logic condition changes on the output, the threshold voltage is also changed. The difference in threshold voltages is called hysteresis.
The basic operation of a Schmitt Trigger is illustrated in FIG. 1. Initially, the output is at logic 0. When the input voltage increases to a high switching threshold voltage VTH+, the output will change states to logic 1. Upon this change, the switching threshold of the Schmitt Trigger circuit will be changed from VTH+ to a lower threshold VTH-. Therefore, in order to switch back to a logic 0 output, the input voltage must fall below the lower threshold VTH-.
There are at least two applications of the Schmitt Trigger. First, when properly used the Schmitt Trigger reduces errors in digital systems due to noise in the input signal. The reduction in errors occurs because once the output has switched and the threshold voltage changed, the input voltage must cross both threshold voltages to produce a logic change on the output. If the magnitude of the noise is less than the hysteresis of the Schmitt Trigger, then the circuit will not respond to the noise. This is illustrated in FIGS. 2a and 2b.
In a second application, the Schmitt Trigger provides a fast transition on the output, even to slow changes on the input. Therefore, the circuit is useful in waveform generation, such as sine wave to square wave converters, and for pulse generation.
2. Description of the Prior Art
CMOS circuits have been used in the past to fabricate Schmitt Triggers. Two general techniques have been employed. The first technique provides circuits that require a very small power supply current. The circuit includes an input stage including a CMOS inverter (i.e., an N channel and P channel transistor with their gates connected together and their drains connected together) and a diode connected MOS transistor. The diode increases the switching threshold voltage of the input inverter for the high switching threshold VTH+, and the diode is shorted to obtain VTH-. The primary disadvantage of such a circuit is that both the low threshold voltage VTH- and the hysteresis are established by process parameters of the N channel and P channel transistors of the CMOS inverter. The combined effects of temperature and process variations produce very large variations in the switching thresholds.
A second type of CMOS Schmitt Trigger circuit employs a circuit having a differential input section (comparator) such as shown in FIG. 3. The differential section includes transistors 10 and 12 having the same conductivity type, with the transistor 10 receiving the input signal at its gate and the transistor 12 receiving a feedback voltage equal to either the low threshold voltage or high threshold voltage at its gate. The drain of the transistor 12 is the output terminal of the circuit. The output drives a switchable voltage divider 14 to provide the proper threshold voltage to the gate of the transistor 12. Because the elements of the differential pair both have the same process parameters, the switching threshold voltages exhibit less sensitivity to process and temperature.
Although the circuit of FIG. 3 is able to provide precise threshold voltages, the differential pair and the voltage divider require a current source. This is provided in the circuit shown in FIG. 3 by bias transistors 16 and 18, each of which is independently biased to provide the necessary current from the power supply VDD. Thus, precision is obtained at the expense of relatively high power requirements.